362 ANNUAL OF SCIENTIFIC DISCOVERY. 



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small, as the rupture of the planet would not probably occur precisely 

 at the time of its equinox. 



Or, also, with the same time of rotation, and a probable mass and 

 density derived from similar sources, we may again obtain the equato- 

 rial diameter, and also the ellipticity, and thence the polar diameter. 



The equatorial diameter thus obtained, if the density assumed be 

 assumed as uniform, will most probably be too great. 



From the mean of two results thus separately obtained, it would at 

 present appear that the equatorial diameter of the planet was about 

 50,000 miles, while its polar diameter was scarcely greater than the 

 thickness of the bright rings of Saturn. 



O O 



ON THE PLANET MARS. 



M. Arago, in a communication made to the French Academy shortly 

 before his death, made the following remarks in reference to the planet 

 Mars. 



After suggesting that the greenish spots visible in Mars probably 

 were produced by a contrast of colors, the learned Academician con- 

 tinued that the red color which covers nearly the whole of Mars' sur- 

 face is real and permanent ; the ancients, even unprovided with 

 telescopes, had attested its existence from the most distant periods of 

 time. Several explanations of this phenomenon have been given ; 

 some suggest that it is caused by ocreous soils or red stones, such as are 

 common on this globe ; Lambert thought it might possibly arise from 

 the vegetation being all red (!) ; others invoked the supposed absorb- 

 ing power of the planet's atmosphere. If Mars has an atmosphere, it 

 does not color the planet. AVhy not ? Because if this atmosphere 

 colored light by absorption, the coloring would be greater on the bor- 

 ders of the planet than on the centre, where the distance to be 

 traversed is less than on the borders ; with the telescope precisely the 

 contrary is observed to take place. This observation restores to the 

 planet its color as inherent in its mass, and at the same time confirms 

 the existence of an atmosphere, giving it however a very different 

 action from the one just mentioned, for it diminishes on the borders of 

 the planet, the natural color of the latter. The admeasurement of 

 the rotation of Mars has constantly presented a great difficulty (an 

 uncertainty of some seconds still prevails relative to the real duration 

 of Mars' rotation), in consequence of the dark spot, (these dark spots 

 have always been used to measure this rotation,) chosen by the observer 

 constantly disappearing before it reached the border of the apparent 

 disk. M. Arago attributes this disappearance to the same atmosphere 

 which interposes and diffuses the light, and whose thickness increases 

 with the obliquity of the beams which traverse it. 



